1. Field of the Invention
The present invention relates to a sealed rectifier for a vehicle which is used in a severe temperature condition.
2. Description of the Related Art
Sealed rectifiers employed in a vehicular alternator, which is mounted in the engine compartment, are subject to temperature changes caused by change in operating conditions of the alternator and the engine. The rectifiers used in such an alternator are required to rectify current amounting to 50-150 amperes (A). Therefore, it is necessary for each of the rectifiers to have wide contact areas connecting a plus-side electrode and the plus-side surface of a semiconductor diode chip and connecting a minus-side electrode and the minus-side surface of the diode chip.
In order to provide sufficient contact areas, a pole-shaped or cup-shaped solid conductive member made of copper, copper alloy or iron alloy is used for the electrode instead of a bonding wire. If such a pole-shaped or cup-shaped electrode made of copper alloy is soldered to the semiconductor diode chip made of silicon however, a big difference in the coefficient of linear expansion would come to an existence between the electrode and the chip. For example, coefficients of the linear expansion of copper and silicon are, respectively, 17 ppm/.degree.C. and 3 ppm/.degree.C.
If the rectifier having the above-stated electrode is used in a severe temperature condition, such as the ambient temperature changes cyclically and widely, the junction of the two members may crack sooner or later.
In order to prevent cracks due to the difference in the linear expansion between the two members, JP-B2-7-120736 proposes to insert an intermediate member made of a material having a medium coefficient of the linear expansion between the chip and the electrode.
However, the addition of the intermediate member increases resistance of the junction and temperature of the junction and, in turn, increases the production cost.
JP-A4-229639 proposes sealing of the chip and the electrode with an insulating member made of epoxy resin. The insulating member contracts after it cools down and applies a pressure to the junction in the direction perpendicular to the junction surface. This structure is intended to assure an electrical connection between the chip and electrode even if the junction cracks.
Although this structure can elongate the life of the rectifier to some extent, such pressure is not sufficient to assure the long time reliability of the rectifier for a vehicular alternator.